Heterostraci
Encyclopedia
Heterostraci is an extinct class
of jawless vertebrate
that lived primarily in marine
and estuary
environments. They arose during the Ordovician
, and all, save for the Psammosteids, became extinct during the late Devonian
. This last group of heterostracans died out in the extinction event at the end of the Devonian.
of their scales.
Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as Lepidaspis
, the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, though, these tiny scales have fused together to form the shield-plates.
The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella."
As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions.
Cyathaspidiformes ("Cup Shields"), and Pteraspidiformes ("Wing Shields"). The Silurian
heterostracan Athenaegis is regarded as being a sister group of both orders, while the Early Devonian Lepidaspis
is regarded as being incertae sedis
, possibly close to the original basal forms.
Cyathaspidiformes is divided into two main groups: the Amphiaspida of Early Devonian Siberia
(grouped together with their relative Ctenaspis
of Canada
), and the Cyathaspidida (grouped together with their relative Nahanbiaspis.
The Pteraspidiformes is divided up into five families
, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae
, and Psammosteidae
.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of jawless vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
that lived primarily in marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
and estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
environments. They arose during the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, and all, save for the Psammosteids, became extinct during the late Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. This last group of heterostracans died out in the extinction event at the end of the Devonian.
Description and anatomy
The Heterostracans differed from other Paleozoic agnathan classes both in the arrangement of their scales, as well as the histologyHistology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
of their scales.
Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as Lepidaspis
Lepidaspis
Lepidaspis serrata is an extinct heterostracan jawless fish from Early Devonian Canada. Its genus name refers to the fact that the shield is composed of hundreds of tiny scales....
, the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, though, these tiny scales have fused together to form the shield-plates.
The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella."
As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions.
Taxonomy
Heterostraca is divided into two main groups, the ordersOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Cyathaspidiformes ("Cup Shields"), and Pteraspidiformes ("Wing Shields"). The Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
heterostracan Athenaegis is regarded as being a sister group of both orders, while the Early Devonian Lepidaspis
Lepidaspis
Lepidaspis serrata is an extinct heterostracan jawless fish from Early Devonian Canada. Its genus name refers to the fact that the shield is composed of hundreds of tiny scales....
is regarded as being incertae sedis
Incertae sedis
, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
, possibly close to the original basal forms.
Cyathaspidiformes is divided into two main groups: the Amphiaspida of Early Devonian Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
(grouped together with their relative Ctenaspis
Ctenaspis
Ctenaspis is an extinct genus of jawless fish from the early Devonian of Europe and North America.species*Ctenaspis kiaeri -Sources:*Vertebrate Fossils and the Evolution of Scientific Concepts by W. Sargeant*BioLib.cz...
of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
), and the Cyathaspidida (grouped together with their relative Nahanbiaspis.
The Pteraspidiformes is divided up into five families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae
Protaspididae
The Protaspididae are an extinct marine family of fish-like vertebrates. The protaspidids existed in the early Devonian period, representing a transitional form between the Pteraspididae and the Psammosteida, bearing the broad head shield shape of the latter, due to a more benthic ...
, and Psammosteidae
Psammosteidae
The Psammosteidae are a family of flattened, benthic jawless vertebrates that lived in marine and estuary environments in Europe, Russia & North America. They arose during the Early Devonian, with the first known genus, Drepanaspis from the Hunsrück lagerstätte...
.